Union Pacific 4017, or UP 4017, is a four-cylinder articulated 4-8-8-4 Big Boy-type steam locomotive owned by Union Pacific Railroad. It was built by ALCO in 1941 along with the other Big Boy locomotives to rectify issues with the preceding 4-6-6-4 "Challenger" class locomotives.

It ran in service hauling freight trains until July 21, 1959, when it was retired along with the other "Big Boy" locomotives.

It is now a static display at the National Railroad Museum in Green Bay, Wisconsin.


  • No. 4017 is one of eight surviving Big Boys.
  • The locomotive was one of the last Big Boys to undergo heavy repairs at Union Pacific's Cheyenne, WY, workshops in July 1959.